1. Field of the Invention
This invention relates to optical-fiber communication systems, and more particularly, to a distortion-compensation circuit for a wideband optical-fiber communication system which can compensate for the distortions in the transmitted signals caused by analog nonlinear devices in the system, allowing the transmitted signals essentially undistorted.
2. Description of Related Art
The optical-fiber systems are now widely used in all areas of communications, such as the cable television (CATV) systems, for simultaneous transmission of multiple frequency-division-multiplexed channels of signals and information. In a CATV system, it is desired that the power be increased so as to allow the broadcast signals to be transmitted to farther places to serve more subscribers, and the bandwidth and number of carriers be increased so as to increase the number of channels that can send more programs to the subscriber via a signal cable link. However, the distortions caused by nonlinear devices in the system are still a problem to the provision of high-quality picture transmissions.
In a CATV system based on multi-carrier analog communications, achieving the objective of high-quality picture transmission requires that the transmitted signals have low noise and distortions. To allow the transmitted signals to have low noise, one method is to increase the power of modulating signals so as to increase the signal-to-noise (S/N) ratio. However, this will also increase the distortions in the transmitted signals caused by the non-linear characteristics of the light-emitting and receiving devices in the system.
In an optical system transporting a single radio-frequency (RF) or microwave carrier, any nonlinear optical or electrical devices will cause the generation of undesired harmonics to the carrier, as depicted in FIG. 1. As shown, when the carrier, which is a sinusoidal signal of a frequency f.sub.0, passes through a nonlinear device 10, the output of the nonlinear device 10 will include an infinite series of harmonics whose frequencies are integral multiples of the frequency f.sub.0 of the input signal (called fundamental frequency). The first-order harmonic has a frequency f.sub.0 which is the same as the input signal; the second-order harmonic has a frequency 2f.sub.0 ; the third-order harmonic has a frequency 3f.sub.0 ; and so forth. The amplitude of these harmonics usually decreases as the order is increased. Normally, the amplitude of the fourth or higher order harmonics are small enough to be negligible. Therefore, typically, only the first three harmonics are significant for consideration.
In a multiple-RF(or microwave)-carrier system, the so-called composite second order (CSO) and composite triple beat (CTB) will arise due to the intermodulation among RF(or microwave) carriers passing through nonlinear devices in the system. The CSO and CTB will cause the so-called intermodulation distortions (IMD) which will deteriorate the quality of the transmitted information. For detailed background information on this, novice readers can refer to IEEE JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 7, No. 11, November 1989.
The CSO and CTB will become more significant as the power or the number of RF(or microwave) carriers used in the system is increased. When the CSO and CTB are increased to a certain degree, the quality of the transmitted video information through the CATV system will be adversely affected. There exists, therefore, a need for a compensation means which can compensate for the CSO/CTB distortions.
A conventional method for compensating for the CSO/CTB distortions is to split the input signal into two parts: a first part of the input signal (hereinafter referred to as primary signal) which is directed to a first path (also called the main path) and a second part of the input signal (hereinafter referred to as secondary signal) which is directed to a second path. In particular, the second path is arranged with a so-called distortion generator that can compensate for the distortions such as CSO or CTB, or both. In the case of compensating for CSO distortion, a CSO distortion generator (which is also called a second-order circuit), such as that disclosed in ROC Publication No. 276842, is arranged on the second path. In a separate case of compensating for CTB distortion, a CTB distortion generator, such as that disclosed in the U.S. Pat. No. 5,210,633, is also arranged on the second path. Further, the final output of the second path and the final output of the first path are combined to obtain the difference therebetween, whereby the undesired CSO or CTB component can be cancelled.
Moreover, in the case of compensating for both of the CSO and CTB distortions, a third path is further added to the first and second paths mentioned above. The circuit configuration to deal with this case is schematically depicted in FIG. 2. As shown, the input signal is first split into two parts: a primary signal which is directed to a first path (main path) 20, and a secondary signal which is directed to a second path 21; and then the secondary signal is further split into a first subpart signal directed to a first subpath on which a CSO distortion generator 22 is arranged and a second subpart signal directed to a second subpath on which a CTB distortion generator 23 is arranged. The output of the CSO distortion generator 22 is used to offset the CSO component in the primary signal, while the output of the CTB distortion generator 23 is used to offset the CTB component in the same. This method can therefore compensate for both of the CSO and CTB distortions in the input signal. Conventional distortion-compensation circuits using this method are disclosed, for example, in the U.S. Pat. No. 5,132,639 and U.S. Pat. No. 5,424,680.
One drawback to the foregoing method, however, is that it requires the use of two separate distortion generators, i.e., one CSO distortion generator and one CTB distortion generator, which will cause the circuit configuration to be too complex. There exists, therefore, a need for a distortion-compensation circuit which includes only one distortion generator that can be variably adjusted to compensate for either a CSO distortion, or a CTB distortion, or both of the CSO and CTB distortions in the input signal.